Maybe we gave a concept of rocks, because there are rocks.

It isn't a question of all sentences of the form "we have a concept of X because X exists" being analytically true. It is a question of having evidence of specific things. The other minds problem is the other minds problem because we all have evidence of our own minds.

If someone is physicalist then they can't assume consciousness a priori. In which case, how can observation of brains and behaviours justify a concept like consciousness?

1 I am aware of my own consciousness

2 my own consciousness must be an outcome of the physical operation of my brain

3 similarly operating brains must be similarly conscious